The thing that I notice most about GTA is that everyone has their own style of playing it.
Some people just play through the storyline, whilst others piss about in the countryside, others roleplay as a cop or gangster, others drive around shooting prostitutes, others go around collecting items, others race and stunt with vehicles, others make mods or videos, and many more I can't even imagine.
GTA San Andreas gave many more opportunities for that than any other GTA game, or even any other game. You could be everything from a mafia boss to a taxi driver to a military scientist in Area 69. You could spend your day chilling on grove street, driving down the open road, robbing casinos or flying over the vast countryside.
GTA IV, in contrast, was much more 2-dimensional. Outside of the storyline, there wasn't much else to do except drive around a city. There weren't many buildings to go in, no countryside to explore, no planes to fly, no destinations really.
It looks like GTA V will change that - I just hope they go all-the-way and give us the best ever sandbox to play in.